1. Field of the Invention
This invention relates to a toner and a microcapsule toner to be used for electrophotography, electrostatic printing, magnetic recording, and the like.
2. Description of the Prior Art
Toners to be used in electrophotography, electrostatic printing, magnetic recording, etc. are materials for formation and recording of images. For example, in electrophotography, there have been known a number of methods as disclosed in U.S. Pat. No. 2,297,691. Generally speaking, a photoconductive material is utilized, an electrical latent image is formed on a photosensitive body by various means, subsequently said latent image is developed by use of a toner, and the developed image, optionally after transferred onto a transfer member such as a paper, fixed by heating under pressurization or a solvent steam to obtain a copied product. Various methods have been proposed for development or fixation of toners and employed as desired.
As the toner to be used in the prior art for this purpose, there have been employed fine powders having coloring materials such as dyes or pigments dispersed in binding materials such as natural or synthetic resins. In general, fine particles have been prepared by blending binding materials with coloring materials, melt mixing the blend at a high temperature, and after cooling crushing the mixture by means of a crushing device utilizing a jet air stream. The toner to be used for the above purpose is intended in the first place to give uniform and stable images. However, the toner obtained according to the above method known in the art can hardly be obtained as essentially uniform particles since the toner is prepared through the mixing operation, the cooling operation and the crushing operation. That is, toners are generally employed with particle sizes of several microns to 30 microns, but it is very difficult to mix completely uniformly coloring materials and binding materials, whereby only ununiform particles can be obtained. For this reason, there have been employed a mixing device having a high shearing force and the method in which coloring materials are processed for improvement of dispersion in binding materials, but these methods proved to be not necessarily satisfactory. One reason is that, although a finely dispersed state may be maintained under heat molten state, phase separation occurs on cooling and therefore the particles as the result of crushing become ununiform. Further, when cooled under a considerably uniform state and subjected to crushing in the subsequent crushing step, even if the crushing force may be exerted uniformly, crushing is liable to occur at the portion which is essentially ununiform, whereby the particles formed in the crushing step cannot be obtained to have uniform shapes and, of course, there are various sizes, with the dispersion states of coloring materials being also diversely different. When such toner particles are practically used, due to such ununiformities possessed by the particles, namely ununiformities in optical properties such as coloring power, hiding power, etc., electrical properties such as electrostatic charges, conductivity, etc. and thermal properties such as melting point, melting heat, etc., no uniform developing characteristic, transfer characteristic or fixing characteristic can be obtained to give inevitably images which are ununiform, unclear or unstable. Also, ununiform shapes possessed by such toners of the prior art include ununiformity in mechanical strength. This leads to the result in practical use of the toner that there occurs changes of the toner due to the change in shapes of the toner, namely insufficient durability characteristic of the toner. In the prior art, for overcoming such problems, it has also been known to add or mix a substance for making uniform the entire mass and also to apply a surface treatment to make uniform the shapes or the characteristics of the particles or a classification treatment to make uniform the particle sizes. However, according to any one of these methods, it has been difficult to obtain satisfactory results.
As a means to overcome these problems, there is also known a method in which microparticles of a mixture of a monomer and a coloring material are prepared and polymerization is carried out under such a state to give directly a toner, as disclosed in Japanese Patent Publication No. 14895/1976. However, the method to prevent some problems involved in this method, namely to prevent completely lowering the toner characteristics caused by incomplete polymers, stabilizers or emulsifiers remaining in the toner, is insufficient and therefore practical application of this method for preparation of the toner was not realizable.
Particularly as a toner for pressure fixing, so called function-separated toner, namely the microcapsule toner is effective, which satisfies both aspects of fixing and developing characteristics at the same time. On the other hand, as core materials for microcapsule toners already known in the art, soft materials are generally employed. As the method for preparation of microcapsule particles with uniform particle sizes by use of soft core materials which can difficultly be crushed by a dry system crushing machine, there have been known in the prior art the two methods as shown below:
(i) Wet system crushing method; and
(ii) Atomizing drying methed.
A wet system crushing method is a method in which formation of core particles is previously performed and subsequently or at the same time shells are formed to effect microencapsulation. Namely, the method comprises applying a dispersing step or emulsifying step (hereinafter called as the first step) to divide previously a core material into relatively smaller particles and an encapsulating step to attach shells thereon (called as the second step). The first step is a step wherein a large amount of an emulsifier is employed, further with addition of a dispersing aid, if required, and uniform microparticles are formed by utilizing a high speed stirrer or an ultrasonic crushing machine. The second step is a step, wherein a shell material is deposited and attached on the surfaces of the core particles after once separating said particles by filtration or continuously. As the method for deposition and attachment on the core particle surfaces, there may be utilized the interfacial polymerization method, the phase separation method or the temperature gradient precipitation method. In some cases, it is also possible to further form an intermediate layer, thereby reinforcing chemically or physical the shell material or the core material. However, according to this method, a large quantity of an emulsifier acting on micropulverization in the first step will necessarily remain on the core material surfaces in the second step subsequently conducted to lower to a great extent the adhesive force of the shell material with by-production of a large number of individual particles formed only of the shell material. As the result, it is difficult to obtain microcapsules with desired uniform particle sizes, which are good in so called mono-separability. Moreover, hygroscopic phenomenon may also occur due to the remaining emulsifier to give deleterious effects on electrophotographic characteristics. For this reason, there is generally performed a pre-treatment prior to the microencapsulation step to remove the emulsifier by washing the micropulverized core material with water or by utilizing an electric dialysis, a semipermeable memberance or an ion exchange resin. However, the microencapsulated particles produced by use of this method will result in decreased yield or increased cost due to cumbersomeness in working operations, thus involving very difficult problems in practice thereof. For producing microcapsule toners according to the wet crushing method, in addition to the above drawbacks, there is also a problem with respect to broadening of particle sizes caused necessarily by mechanical stirring.
On the other hand, the method of producing microcapsule particles by utilization of the atomizing drying method is a method in which a substance for core material and a substance for shell material, which are previously kneaded with each other or dispersed in a medium, are discharged through a nozzle under an atmospheric condition to have the shell material attached on the core particle surfaces. However, the microcapsule particles obtained by this method have generally a wide particle size distribution and the particles are liable to become coarse.
The microcapsule particles are polymeric vessels having sizes of about several microns to some hundred microns and, through utilization of the function to protect the content or to control release of the content, have been presently applied in various uses, not only in the aforesaid toner, but also in commercial products such as carbonless copying papers, rapid- or slow-acting pharmaceuticals, catalysts or rust preventives. In view of their wide utility values, the abovementioned usages are only a part of their applications, and a great development is expected for such microcapsule particles.